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You know that dating methods are based on evolutionary assumptions No they aren't. Absolute dating techniques are based on physics (e.g. radioactive decay) and processes of physical geology (e.g. the cooling of magmas). Not a single thing to do with evolution, or anything related to biology. The modern scheme of relative dating (i.e. the geological column with it's major divisions) was worked out by geologists who were creationists to a man, and was in place decades before Darwin published. How can something that was done entirely be creationists be based on "evolutionary assumptions"?

Flaws in dating the earth as ancient by Alexander R. Williams

In 1986 the world’s leading science journal, Nature, announced that the most ancient rock crystals on earth, according to isotope dating methods, are 4.3 billion years old and come from Jack Hills in Western Australia.

W. Compston and R.T. Pidgeon (Nature 321:766–769, 1986) obtained 140 zircon crystals from a single rock unit and subjected them to uranium/uranium concordia (U/U)1 and uranium/thorium concordia (U/Th)2 dating methods. One crystal showed a U/U date of 4.3 billion years, and the authors therefore claimed it to be the oldest rock crystal yet discovered.

A serious problem here is that all 140 crystals from the same rock unit gave statistically valid information about that rock unit.3 No statistician could ever condone a method which selected one value and discarded all the other 139. In fact, the other 139 crystals show such a confusion of information that a statistician could only conclude that no sensible dates could be extracted from the data.

A further problem is that the 4.3 billion-year-old zircon, dated according to the U/U method, was identified by the U/Th method to be undatable. An unbiased observer would be forced to admit that this contradiction prevents any conclusion as to the age of the crystal. But these authors reached their conclusion by ignoring the contradictory data! If a scientist in any other field did this he would never be allowed to publish it. Yet here we have it condoned by the top scientific journal in the world.

This is not an isolated case. I selected it because it was identified by the journal editors as a significant advance in knowledge. Another example is the work of F.A. Podosek, J. Pier, O. Nitoh, S. Zashu, and M. Ozima (Nature 334:607–609, 1988). They found what might have been the world’s oldest rock crystals, but unfortunately they were too old!

They extracted diamonds from rocks in Zaire and found by the potassium-argon method that they (the diamonds) were six billion years old. But the earth is supposed to be only 4.5 billion years old. So Podosek and friends decided they must be wrong. They admitted, however, that if the date had not been contradicted by the ‘known’ age of the earth, they would have accepted it as valid.

http://www.answersingenesis.org/creation/v18/i1/earth.asp

BTW, diamonds are not suitable subjects for K-Ar dating. The key assumption behind this method is that all Argon degassed from the subject rock the last time it was in a molten state. This is usually a safe assumption, but various studies have revealed exceptions. An example is "pillow lavas" which cool very quickly (under water) and therefore don't leave insufficient time for all the Argon in the magma to escape.

Studies have actually been done confirming this, e.g. dating pillow lavas at progressive points from the outside in. Outer layers give older "dates," and the age gets progressively "younger" toward the interior of the lava which cooled more slowly.

Ironically experiments like this -- where geologists are explicitly, intentionally and rigorously testing the assumptions on which the various radiometric dating techniques are based -- are regularly, and dishonestly, cited by creationists as "anomalous" dates which "prove" that geologists don't question their assumptions!

Anyway, diamonds appear to be another example that don't meet the necessary criteria for this particular dating method. Instead of forming in a crustal magma melt they actually form in the upper mantle, which is believed on good evidence to hold a rich collection of noble gases, including argon. The lattice structure of the diamond is ideal for trapping such gases.